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diff --git a/include/ruby/internal/encoding/string.h b/include/ruby/internal/encoding/string.h
new file mode 100644
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+++ b/include/ruby/internal/encoding/string.h
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+#ifndef RUBY_INTERNAL_ENCODING_STRING_H              /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY_INTERNAL_ENCODING_STRING_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed  with   either  `RBIMPL`   or  `rbimpl`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries.  They could be written in C++98.
+ * @brief      Routines to manipulate encodings of strings.
+ */
+
+#include "ruby/internal/dllexport.h"
+#include "ruby/internal/value.h"
+#include "ruby/internal/encoding/encoding.h"
+#include "ruby/internal/attr/nonnull.h"
+#include "ruby/internal/intern/string.h" /* rbimpl_strlen */
+
+RBIMPL_SYMBOL_EXPORT_BEGIN()
+
+/**
+ * Identical to rb_str_new(), except it additionally takes an encoding.
+ *
+ * @param[in]  ptr             A memory region of `len` bytes length.
+ * @param[in]  len             Length  of `ptr`,  in bytes,  not including  the
+ *                             terminating NUL character.
+ * @param[in]  enc             Encoding of `ptr`.
+ * @exception  rb_eNoMemError  Failed to allocate `len+1` bytes.
+ * @exception  rb_eArgError    `len` is negative.
+ * @return     An instance  of ::rb_cString,  of `len`  bytes length,  of `enc`
+ *             encoding, whose contents are verbatim copy of `ptr`.
+ * @pre        At  least  `len` bytes  of  continuous  memory region  shall  be
+ *             accessible via `ptr`.
+ * @note       `enc` can be a  null pointer.  It can also be  seen as a routine
+ *             identical to rb_usascii_str_new() then.
+ */
+VALUE rb_enc_str_new(const char *ptr, long len, rb_encoding *enc);
+
+RBIMPL_ATTR_NONNULL((1))
+/**
+ * Identical to  rb_enc_str_new(), except  it assumes the  passed pointer  is a
+ * pointer  to a  C string.  It can  also  be seen  as a  routine identical  to
+ * rb_str_new_cstr(), except it additionally takes an encoding.
+ *
+ * @param[in]  ptr             A C string.
+ * @param[in]  enc             Encoding of `ptr`.
+ * @exception  rb_eNoMemError  Failed to allocate memory.
+ * @return     An instance  of ::rb_cString, of `enc`  encoding, whose contents
+ *             are verbatim copy of `ptr`.
+ * @pre        `ptr` must not be a null pointer.
+ * @pre        Because `ptr` is  a C string it  makes no sense for  `enc` to be
+ *             something like UTF-32.
+ * @note       `enc` can be a  null pointer.  It can also be  seen as a routine
+ *             identical to rb_usascii_str_new_cstr() then.
+ */
+VALUE rb_enc_str_new_cstr(const char *ptr, rb_encoding *enc);
+
+/**
+ * Identical to rb_enc_str_new(),  except it takes a C string  literal.  It can
+ * also  be seen  as  a  routine identical  to  rb_str_new_static(), except  it
+ * additionally takes an encoding.
+ *
+ * @param[in]  ptr           A C string literal.
+ * @param[in]  len           `strlen(ptr)`.
+ * @param[in]  enc           Encoding of `ptr`.
+ * @exception  rb_eArgError  `len` out of range of `size_t`.
+ * @pre        `ptr` must be a C string constant.
+ * @return     An instance  of ::rb_cString,  of `enc` encoding,  whose backend
+ *             storage is the passed C string literal.
+ * @warning    It is  a very  bad idea to  write to a  C string  literal (often
+ *             immediate  SEGV shall  occur).  Consider  return values  of this
+ *             function be read-only.
+ * @note       `enc` can be a  null pointer.  It can also be  seen as a routine
+ *             identical to rb_usascii_str_new_static() then.
+ */
+VALUE rb_enc_str_new_static(const char *ptr, long len, rb_encoding *enc);
+
+/**
+ * Identical to rb_enc_str_new(),  except it returns a "f"string.   It can also
+ * be seen as a routine  identical to rb_interned_str(), except it additionally
+ * takes an encoding.
+ *
+ * @param[in]  ptr           A memory region of `len` bytes length.
+ * @param[in]  len           Length  of  `ptr`,  in bytes,  not  including  the
+ *                           terminating NUL character.
+ * @param[in]  enc           Encoding of `ptr`.
+ * @exception  rb_eArgError  `len` is negative.
+ * @return     A  found or  created instance  of ::rb_cString,  of `len`  bytes
+ *             length, of `enc` encoding, whose  contents are identical to that
+ *             of `ptr`.
+ * @pre        At  least  `len` bytes  of  continuous  memory region  shall  be
+ *             accessible via `ptr`.
+ * @note       `enc` can be a null  pointer.
+ */
+VALUE rb_enc_interned_str(const char *ptr, long len, rb_encoding *enc);
+
+RBIMPL_ATTR_NONNULL((1))
+/**
+ * Identical to rb_enc_str_new_cstr(),  except it returns a  "f"string.  It can
+ * also be  seen as  a routine identical  to rb_interned_str_cstr(),  except it
+ * additionally takes an encoding.
+ *
+ * @param[in]  ptr           A memory region of `len` bytes length.
+ * @param[in]  enc           Encoding of `ptr`.
+ * @return     A found  or created instance  of ::rb_cString of `enc` encoding,
+ *             whose contents are identical to that of `ptr`.
+ * @pre        At  least  `len` bytes  of  continuous  memory region  shall  be
+ *             accessible via `ptr`.
+ * @note       `enc` can be a null  pointer.
+ */
+VALUE rb_enc_interned_str_cstr(const char *ptr, rb_encoding *enc);
+
+/**
+ * Counts  the number  of characters  of the  passed string,  according to  the
+ * passed encoding.   This has to be  complicated.  The passed string  could be
+ * invalid and/or broken.   This routine would scan from the  beginning til the
+ * end, byte by byte, to seek out character boundaries.  Could be super slow.
+ *
+ * @param[in]  head  Leftmost pointer to the string.
+ * @param[in]  tail  Rightmost pointer to the string.
+ * @param[in]  enc   Encoding of the string.
+ * @return     Number of characters exist in  `head` .. `tail`.  The definition
+ *             of "character" depends on the passed `enc`.
+ */
+long rb_enc_strlen(const char *head, const char *tail, rb_encoding *enc);
+
+/**
+ * Queries the n-th character.  Like  rb_enc_strlen() this function can be fast
+ * or slow depending on the contents.   Don't expect characters to be uniformly
+ * distributed across the entire string.
+ *
+ * @param[in]  head  Leftmost pointer to the string.
+ * @param[in]  tail  Rightmost pointer to the string.
+ * @param[in]  nth   Requested index of characters.
+ * @param[in]  enc   Encoding of the string.
+ * @return     Pointer  to  the first  byte  of  the  character that  is  `nth`
+ *             character  ahead  of `head`,  or  `tail`  if  there is  no  such
+ *             character (OOB  etc).  The definition of  "character" depends on
+ *             the passed `enc`.
+ */
+char *rb_enc_nth(const char *head, const char *tail, long nth, rb_encoding *enc);
+
+/**
+ * Identical to rb_enc_get_index(), except the return type.
+ *
+ * @param[in]  obj            Object in question.
+ * @exception  rb_eTypeError  `obj` is incapable of having an encoding.
+ * @return     `obj`'s encoding.
+ */
+VALUE rb_obj_encoding(VALUE obj);
+
+/**
+ * Identical to rb_str_cat(), except it additionally takes an encoding.
+ *
+ * @param[out]  str                 Destination object.
+ * @param[in]   ptr                 Contents to append.
+ * @param[in]   len                 Length of `src`, in bytes.
+ * @param[in]   enc                 Encoding of `ptr`.
+ * @exception   rb_eArgError        `len` is negative.
+ * @exception   rb_eEncCompatError  `enc` is not compatible with `str`.
+ * @return      The passed `dst`.
+ * @post        The  contents  of  `ptr`  is copied,  transcoded  into  `dst`'s
+ *              encoding, then pasted into `dst`'s end.
+ */
+VALUE rb_enc_str_buf_cat(VALUE str, const char *ptr, long len, rb_encoding *enc);
+
+/**
+ * Encodes the passed code point into a series of bytes.
+ *
+ * @param[in]  code             Code point.
+ * @param[in]  enc              Target encoding scheme.
+ * @exception  rb_eRangeError  `enc` does not glean `code`.
+ * @return     An  instance  of ::rb_cString,  of  `enc`  encoding, whose  sole
+ *             contents is `code` represented in `enc`.
+ * @note       No way to encode code points bigger than UINT_MAX.
+ *
+ * @internal
+ *
+ * In  other languages,  APIs like  this  one could  be seen  as the  primitive
+ * routines where encodings' "encode" feature are implemented.  However in case
+ * of  Ruby this  is not  the primitive  one.  We  directly manipulate  encoded
+ * strings.  Encoding conversion routines  transcode an encoded string directly
+ * to another one; not via a code point array.
+ */
+VALUE rb_enc_uint_chr(unsigned int code, rb_encoding *enc);
+
+/**
+ * Identical  to   rb_external_str_new(),  except  it  additionally   takes  an
+ * encoding.  However the  whole point of rb_external_str_new() is  to encode a
+ * string  into default  external encoding.   Being able  to specify  arbitrary
+ * encoding just ruins the designed purpose the function meseems.
+ *
+ * @param[in]  ptr           A memory region of `len` bytes length.
+ * @param[in]  len           Length  of  `ptr`,  in bytes,  not  including  the
+ *                           terminating NUL character.
+ * @param[in]  enc           Target encoding scheme.
+ * @exception  rb_eArgError  `len` is negative.
+ * @return     An instance  of ::rb_cString.  In case  encoding conversion from
+ *             "default  internal" to  `enc` is  fully defined  over the  given
+ *             contents, then the  return value is a string  of `enc` encoding,
+ *             whose contents are the converted  ones.  Otherwise the string is
+ *             a junk.
+ * @warning    It doesn't raise on a conversion failure and silently ends up in
+ *             a  corrupted  output.  You  can  know  the failure  by  querying
+ *             `valid_encoding?` of the result object.
+ *
+ * @internal
+ *
+ * @shyouhei has  no idea why  this one does  not follow the  naming convention
+ * that  others obey.   It  seems to  him  that this  should  have been  called
+ * `rb_enc_external_str_new`.
+ */
+VALUE rb_external_str_new_with_enc(const char *ptr, long len, rb_encoding *enc);
+
+/**
+ * Identical to rb_str_export(), except it additionally takes an encoding.
+ *
+ * @param[in]  obj            Target object.
+ * @param[in]  enc            Target encoding.
+ * @exception  rb_eTypeError  No implicit conversion to String.
+ * @return     Converted ruby string of `enc` encoding.
+ */
+VALUE rb_str_export_to_enc(VALUE obj, rb_encoding *enc);
+
+/**
+ * Encoding conversion main routine.
+ *
+ * @param[in]  str   String to convert.
+ * @param[in]  from  Source encoding.
+ * @param[in]  to    Destination encoding.
+ * @return     A copy of `str`, with conversion from `from` to `to` applied.
+ * @note       `from` can be a null pointer.  `str`'s encoding is taken then.
+ * @note       `to` can be a null pointer.  No-op then.
+ */
+VALUE rb_str_conv_enc(VALUE str, rb_encoding *from, rb_encoding *to);
+
+/**
+ * Identical  to rb_str_conv_enc(),  except  it additionally  takes IO  encoder
+ * options.  The extra arguments  can be constructed using io_extract_modeenc()
+ * etc.
+ *
+ * @param[in]  str      String to convert.
+ * @param[in]  from     Source encoding.
+ * @param[in]  to       Destination encoding.
+ * @param[in]  ecflags  A set of enum ::ruby_econv_flag_type.
+ * @param[in]  ecopts   Optional hash.
+ * @return     A copy of `str`, with conversion from `from` to `to` applied.
+ * @note       `from` can be a null pointer.  `str`'s encoding is taken then.
+ * @note       `to` can be a null pointer.  No-op then.
+ * @note       `ecopts` can be  ::RUBY_Qnil, which is equivalent  to passing an
+ *             empty hash.
+ */
+VALUE rb_str_conv_enc_opts(VALUE str, rb_encoding *from, rb_encoding *to, int ecflags, VALUE ecopts);
+
+/**
+ * @private
+ *
+ * This is an implementation detail of rb_enc_str_coderange().  Don't use this
+ * directly.
+ **/
+int rbimpl_enc_str_coderange_scan(VALUE str);
+
+/**
+ * Scans the passed string to collect  its code range.  Because a Ruby's string
+ * is mutable, its contents  change from time to time; so  does its code range.
+ * A  long-lived string  tends  to fall  back to  ::RUBY_ENC_CODERANGE_UNKNOWN.
+ * This API scans it and re-assigns a fine-grained code range constant.
+ *
+ * @param[out]  str  A string.
+ * @return      An enum ::ruby_coderange_type.
+ */
+int rb_enc_str_coderange(VALUE str);
+
+/**
+ * Scans the passed string to collect  its code range.  Because a Ruby's string
+ * is mutable, its contents  change from time to time; so  does its code range.
+ * A  long-lived string  tends  to fall  back to  ::RUBY_ENC_CODERANGE_UNKNOWN.
+ * This API scans it and re-assigns a fine-grained code range constant.
+ *
+ * @param[out]  str  A string.
+ * @return      An enum ::ruby_coderange_type.
+ */
+static inline int
+rb_enc_str_coderange_inline(VALUE str)
+{
+    int cr = ENC_CODERANGE(str);
+    if (cr == ENC_CODERANGE_UNKNOWN) {
+        cr = rbimpl_enc_str_coderange_scan(str);
+    }
+    return cr;
+}
+
+#define rb_enc_str_coderange rb_enc_str_coderange_inline
+
+/**
+ * Scans the passed string until it finds something odd.  Returns the number of
+ * bytes scanned.  As the name implies this is suitable for repeated call.  One
+ * of its application is `IO#readlines`.   The method reads from its receiver's
+ * read buffer, maybe more than once,  looking for newlines.  But "newline" can
+ * be different among encodings.  This API is used to detect broken contents to
+ * properly mark them as such.
+ *
+ * @param[in]   str  String to scan.
+ * @param[in]   end  End of `str`.
+ * @param[in]   enc  `str`'s encoding.
+ * @param[out]  cr   Return buffer.
+ * @return      Distance between `str` and first such byte where broken.
+ * @post        `cr` has the code range type.
+ */
+long rb_str_coderange_scan_restartable(const char *str, const char *end, rb_encoding *enc, int *cr);
+
+/**
+ * Queries if  the passed string  is "ASCII only".  An  ASCII only string  is a
+ * string  who doesn't  have any  non-ASCII  characters at  all.  This  doesn't
+ * necessarily mean the string is in  ASCII encoding.  For instance a String of
+ * CP932 encoding can quite much be ASCII only, depending on its contents.
+ *
+ * @param[in]  str  String in question.
+ * @retval     1    It doesn't have non-ASCII characters.
+ * @retval     0    It has characters that are out of ASCII.
+ */
+int rb_enc_str_asciionly_p(VALUE str);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Looks for the passed string in the passed buffer.
+ *
+ * @param[in]  x          Query string.
+ * @param[in]  m          Number of bytes of `x`.
+ * @param[in]  y          Buffer that potentially includes `x`.
+ * @param[in]  n          Number of bytes of `y`.
+ * @param[in]  enc        Encoding of both `x` and `y`.
+ * @retval     -1         Not found.
+ * @retval     otherwise  Found index in `y`.
+ * @note       This API can match at a non-character-boundary.
+ */
+long rb_memsearch(const void *x, long m, const void *y, long n, rb_encoding *enc);
+
+/** @cond INTERNAL_MACRO */
+RBIMPL_ATTR_NONNULL(())
+static inline VALUE
+rbimpl_enc_str_new_cstr(const char *str, rb_encoding *enc)
+{
+    long len = rbimpl_strlen(str);
+
+    return rb_enc_str_new_static(str, len, enc);
+}
+
+#define rb_enc_str_new(str, len, enc)           \
+    ((RBIMPL_CONSTANT_P(str) &&                 \
+      RBIMPL_CONSTANT_P(len) ?                  \
+      rb_enc_str_new_static:                    \
+      rb_enc_str_new) ((str), (len), (enc)))
+
+#define rb_enc_str_new_cstr(str, enc)           \
+    ((RBIMPL_CONSTANT_P(str)  ?                 \
+      rbimpl_enc_str_new_cstr :                 \
+      rb_enc_str_new_cstr) ((str), (enc)))
+
+/** @endcond */
+
+RBIMPL_SYMBOL_EXPORT_END()
+
+#endif /* RUBY_INTERNAL_ENCODING_STRING_H */